Foamed polymers have widespread application in areas such as insulation, flooring, carpet backing, wallpaper, packaging, general consumer goods, toys, and furniture. Although nearly all thermoplastic polymers may be foamed, only a few have gained significant commercial importance. One popular resin for foaming is polystyrene. Polystyrene is used in forming insulation, cushioning, packaging, and a variety of molded products. The popularity of polystyrene stems from its low density, good insulating qualities and its tendency to retain blowing agent which may be added to polystyrene resin for subsequent expansion of impregnated resin into foam. Such post expansion processes typically involve shipping unexpanded pellets of resin impregnated with blowing agent to various purchasers and manufacturers who then expand the pellets into appropriate forms by heating. The disadvantages associated with foamed polystyrene are that it is brittle and often breaks or fractures when subjected to moderate or even light forces.
Polyethylene would be desirable in many of the above applications in view of its relative strength, however polyethylene does not adequately retain blowing agent. The loss of blowing agent from impregnated resin thus presents environmental hazards and health concerns. Moreover, rapid loss of blowing agent from unexpanded polyethylene resin precludes post expansion processes. A polyethylene foam also has a relatively high thermal conductivity. It does not retain blowing agent which provides low thermal conductivity. It is, therefore, a poor thermal insulator.
Polyethylene terephthalate (PET) is recognized as a tough, versatile thermoplastic polyester often used for beverage bottles, food trays and custom containers. PET foams have a relatively low thermal conductivity and high barrier or retention qualities. However, PET is very difficult to foam after extruding.
When foaming polyesters such as PET, it is found that a relatively large amount of blowing agent must be used to stabilize or cool the solidifying foam structure. It is desirable to rapidly cool and solidify the foam thereby stabilizing the expanded bubbles. The use of large amounts of blowing agent assists in the cooling of the foam. As the blowing agent dispersed throughout the impregnated resin vaporizes, heat is absorbed from the foam surroundings to supply the heat of vaporization for the blowing agent. Cooling of the foam also occurs via adiabatic expansion of the blowing agent within the expanding foam structure.
Most conventional volatile organic blowing agents have relatively low solubilities in polyester resins, making it difficult to add the large amounts often required as described above. Blowing agents having high boiling points may be dissolved in polyester resins in sufficient amounts but readily diffuse out of the foam structure causing foam shrinkage. There is a need for a blowing agent which may be added to a polyester resin in relatively large amounts which will remain in the foam structure to a degree such that substantial foam shrinkage will not occur.
When extruding and expanding polyesters such as PET, it is difficult to obtain a low density foam of large cross section since PET foams tend to prefoam before passing through a die orifice, thus requiring small orifice sizes to be used. There is a need for an extrusion expansion process which yields lightweight foams having relatively large cross sections.
Thus, there is a continuing need for a low density polymeric foam having high barrier or retention qualities, which is less brittle and frangible than polystyrene foam, which is particularly extrudable, readily processable and amenable to a variety of foaming processes.